The letters document the development of crucial aspects of Einstein's general theory of relativity, including all three predictions that were eventually verified by experiments or observations. One letter includes what may be the first illustration of the curvature of light, an astro-physical phenomenon predicted by the theory. The letters also reveal Freundlich's role as a disciple, perhaps the first scientist to become thoroughly acquainted with the fundamental principle of Einstein's theory, and as the first person to try to provide experimental proof of the predicted consequences of the general theory.